A motor start circuit to be used in refrigerator or air-conditioner motors is shown in FIG. 11. In that figure, a positive temperature coefficient of resistivity (PTC) thermistor 11 is serially connected with the start winding S of a motor 10 that has a start winding S and a main winding M. An overload protection device 12 is connected to common terminal C of start winding S and main winding M. PTC thermistor 11 has a low resistance at normal ambient temperature when the motor has first been started, with a result that a sufficient amount of start current flows through start winding S to start the motor.
After start-up of the motor, PTC thermistor 11 generates heat due to electric current that flows through it heating up the thermistor, with a consequence that the resistance of the PTC thermistor rises suddenly, bringing about a state of high resistance and maintaining a balanced state with a current of several tens of milli-amperes. In the event of an overload operation or the like of motor 10, overload protection device 12 opens the circuit through common terminal C in response to elevated temperature caused by the excess current and/or the temperature of the winding. Three air-tight terminals are provided at the top of the shell of sealed compressors (which will hereafter be referred to as terminal pins) for an external interface for connection to start winding S, main winding M and common terminal C. The motor start relay accommodates PTC thermistor 11 in an insulated housing and spring terminals are biased against the electrode faces of PTC thermistor 11 as well as having parts which grip onto the terminal pins.
It is known to provide a failsafe mechanism in a motor start relay to deal with breaking of a PTC thermistor element. Reference may be had to Japanese Patent No. 2,891,179, a figure of which is shown in FIG. 12 of this application, for an example of such a mechanism. The positive temperature thermistor device that has been described in that patent has a first spring contact member 40 and a first positioning protrusion 56 engaging first electrode 38 of a positive temperature thermistor 35 that is accommodated in a casing 32 and a second spring contact member 43 and a second positioning protrusion 57 engaging the opposing second electrode 39.
The first spring member 40 and the second spring contact member 43 are located along the direction of an inclined line relative to the face of thermistor 35 and the first positioning protrusion 56 and the second positioning protrusion 57 are located in the direction of another inclined line relative to the face of the thermistor. The first spring contact member 40 is located adjacent to the outer periphery on one face further from the center than the second positioning protrusion 57 on the other face. Likewise, the second spring contact member 43 is located adjacent to the outer periphery on the other face further from the center than the first positioning protrusion 56 on the said one face.
As a result of what has been described above, the direction of the moments acting on thermistor 35 as a result of the spring action of the first and second spring contact members 40 and 43 relative to protrusions 56 and 57 are as indicated by arrows 58 and 59. Angled surfaces 60 and 61 are formed on the outer peripheries of the first and second positioning protrusions 56 and 57.
If the positive temperature thermistor 35 cracks and is damaged, for example, as the result of an arc, the broken parts are shifted in a direction away from each other because of the spring action of the first and second spring contact members 40 and 43, thereby preventing any possible short-circuiting or molten deposition of the broken parts. Thus, a positive action will open the circuit.
Nevertheless, the positive temperature thermistor device as shown in the above referenced part is subject to the following limitations.
In view of the fact that, according to the failsafe mechanism shown in FIG. 12, the positive temperature thermistor 35 is fixed by using the first and second spring contact members 40 and 43 as the force application points and the first and second offset positioning protrusions 56 and 57 as the fulcrums, the first and second positioning protrusions 56 and 57 will always be in contact with the electrode of the positive temperature thermistor 35 during operation when the temperature of the thermistor is high.
In the case where the first and second positioning protrusions 56 and 57 are formed integrally with the housing, it is necessary for the material used for the housing be formed of resin that has a high level of resistance to heat.
Moreover, the positive temperature thermistor 35 is inserted from above the case 32 (in a direction which is perpendicular to the face of the sheet of the drawing), with a result that the first and second spring contact pieces 40 and 43 will be extending into the space in which the positive temperature thermistor 35 is to be inserted. Accordingly, insertion of the positive temperature thermistor 35 is difficult and the spring contact members and positive temperature thermistor will have to be assembled by using jigs.